Index Construction

1. Index Construction Adapted from Lectures by Prabhakar Raghavan (Yahoo and Stanford) and Christopher Manning (Stanford) L06IndexConstruction

2. Index construction • How do we construct an index? • What strategies can we use with limited main memory? • Our Sample Corpus • Number of docs = n = 1M • Each doc has 1K terms • Number of distinct terms = m = 500K • 667 million postings entries

3. Recall index construction • Documents are parsed to extract words and these are saved with the Document ID. Doc 1 Doc 2 I did enact Julius Caesar I was killed i' the Capitol; Brutus killed me. So let it be with Caesar. The noble Brutus hath told you Caesar was ambitious

4. After all documents have been parsed the inverted file is sorted by terms. Key step We focus on this sort step. We have 667M items to sort.

5. Index construction • As we build up the index, cannot exploit compression tricks • Parse docs one at a time. • Final postings for any term – incomplete until the end. • (actually you can exploit compression, but this becomes a lot more complex) • At 10-12 bytes per postings entry, demands several temporary gigabytes

6. System parameters for design • Disk seek ~ 10 milliseconds • Block transfer from disk ~ 1 microsecond per byte (following a seek) • All other ops ~ 10 microseconds • E.g., compare two postings entries and decide their merge order

7. Bottleneck • Parse and build postings entries one doc at a time • Now sort postings entries by term (then by doc within each term) • Doing this with random disk seeks would be too slow – must sort N=667M records If every comparison took 2 disk seeks, and N items could be sorted with N log2N comparisons, how long would this take?

8. Sorting with fewer disk seeks • 12-byte (4+4+4) records (term, doc, freq). • These are generated as we parse docs. • Must now sort 667M such 12-byte records by term. • Define a Block ~ 10M such records • can “easily” fit a couple into memory. • Will have 64 such blocks to start with. • Will sort within blocks first, then merge the blocks into one long sorted order.

9. Sorting 64 blocks of 10M records • First, read each block and sort within: • Quicksort takes 2N ln N expected steps • In our case 2 x (10M ln 10M) steps • Exercise: estimate total time to read each block from disk and quicksort it. • 64 times this estimate - gives us 64 sorted runs of 10M records each. • Need 2 copies of data on disk, throughout.

10. 1 3 2 4 Merging 64 sorted runs • Merge tree of log264= 6 layers. • During each layer, read into memory runs in blocks of 10M, merge, write back. 2 1 Merged run. 3 4 Runs being merged. Disk

11. Merge tree 1 run … ? 2 runs … ? 4 runs … ? 8 runs, 80M/run 16 runs, 40M/run … 32 runs, 20M/run Bottom level of tree. Sorted runs. … 1 2 63 64

12. Merging 64 runs • Time estimate for disk transfer: • 6 x (64runs x 120MB x 10-6sec) x 2 ~ 25hrs. Disk block transfer time.Why is this an Overestimate? Work out how these transfers are staged, and the total time for merging. # Layers in merge tree Read + Write

13. Exercise - fill in this table Step Time 1 64 initial quicksorts of 10M records each Read 2 sorted blocks for merging, write back 2 3 Merge 2 sorted blocks ? 4 Add (2) + (3) = time to read/merge/write 5 64 times (4) = total merge time

14. Large memory indexing • Suppose instead that we had 16GB of memory for the above indexing task. • Exercise: What initial block sizes would we choose? What index time does this yield?

15. Distributed indexing • For web-scale indexing (don’t try this at home!): must use a distributed computing cluster • Individual machines are fault-prone • Can unpredictably slow down or fail • How do we exploit such a pool of machines?

16. Distributed indexing • Maintain a master machine directing the indexing job – considered “safe”. • Break up indexing into sets of (parallel) tasks. • Master machine assigns each task to an idle machine from a pool.

17. Parallel tasks • We will use two sets of parallel tasks • Parsers • Inverters • Break the input document corpus into splits • Each split is a subset of documents • Master assigns a split to an idle parser machine • Parser reads a document at a time and emits (term, doc) pairs

18. Parallel tasks • Parser writes pairs into j partitions • Each for a range of terms’ first letters • (e.g., a-f, g-p, q-z) – here j=3. • Now to complete the index inversion

19. Data flow Master assign assign Postings Parser Inverter a-f g-p q-z a-f Parser a-f g-p q-z Inverter g-p Inverter splits q-z Parser a-f g-p q-z

20. Inverters • Collect all (term, doc) pairs for a partition • Sorts and writes to postings list • Each partition contains a set of postings Above process flow a special case of MapReduce.

21. Dynamic indexing • Docs come in over time • postings updates for terms already in dictionary • new terms added to dictionary • Docs get deleted

22. Simplest approach • Maintain “big” main index • New docs go into “small” auxiliary index • Search across both, merge results • Deletions • Invalidation bit-vector for deleted docs • Filter docs output on a search result by this invalidation bit-vector • Periodically, re-index into one main index

23. Index on disk vs. memory • Most retrieval systems keep the dictionary in memory and the postings on disk • Web search engines frequently keep both in memory • massive memory requirement • feasible for large web service installations • less so for commercial usage where query loads are lighter

24. Indexing in the real world • Typically, don’t have all documents sitting on a local filesystem • Documents need to be spidered • Could be dispersed over a WAN with varying connectivity • Must schedule distributed spiders • Have already discussed distributed indexers • Could be (secure content) in • Databases • Content management applications • Email applications

25. Content residing in applications • Mail systems/groupware, content management contain the most “valuable” documents • http often not the most efficient way of fetching these documents - native API fetching • Specialized, repository-specific connectors • These connectors also facilitate document viewing when a search result is selected for viewing

26. Secure documents • Each document is accessible to a subset of users • Usually implemented through some form of Access Control Lists (ACLs) • Search users are authenticated • Query should retrieve a document only if user can access it • So if there are docs matching your search but you’re not privy to them, “Sorry no results found” • E.g., as a lowly employee in the company, I get “No results” for the query “salary roster”

27. Users in groups, docs from groups • Index the ACLs and filter results by them • Often, user membership in an ACL group verified at query time – slowdown Documents Users 0/1 0 if user can’t read doc, 1 otherwise.

28. “Rich” documents • (How) Do we index images? • Researchers have devised Query Based on Image Content (QBIC) systems • “show me a picture similar to this orange circle” • (see, vector space retrieval) • In practice, image search usually based on meta-data such as file name e.g., monalisa.jpg • New approaches exploit social tagging • E.g., flickr.com

29. Passage/sentence retrieval • Suppose we want to retrieve not an entire document matching a query, but only a passage/sentence - say, in a very long document • Can index passages/sentences as mini-documents – what should the index units be? • This is the subject of XML search